A foam blow-molding process is carried out by extruding a thermoplastic resin containing a foaming agent added thereto into an atmosphere as a parison and inserting the parison between split mold blocks (e.g., refer to Patent Document 1).
As a blow-molded foam product obtained by such a process, there has been known one that principally contains a polypropylene-based resin having predetermined physical properties (e.g., refer to Patent Document 2).
In such a blow-molded foam product, however, when being released to the atmosphere, foamed cells in the parison are abruptly expanded to make cells larger. Occasionally, there is a possibility that the cells are ruptured.
In order to overcome this disadvantage, there has been examined a process for making foamed cells finer in diameter while maintaining a high expansion ratio.
For example, there has been mentioned a foam duct molded by foam blow-molding in which a supercritical fluid is added as a foaming agent (e.g., refer to Patent Document 3).    Patent Document 1: JP 63-309434 A    Patent Document 2: Japanese Patent No. 3745960    Patent Document 3: JP 2005-241157 A